Free-cutting steel and method of producing same

Abstract

Provided is a free-cutting steel that, despites not containing Pb, has machinability by cutting higher than or equal to that of a low carbon sulfur-lead composite free-cutting steel. A free-cutting steel comprises: a chemical composition that contains, in mass %, C: less than 0.09%, Mn: 0.50% to 1.50%, S: 0.250% to 0.600%, O: more than 0.010% and 0.050% or less, and Cr: 0.50% to 1.50%, with a balance consisting of Fe and inevitable impurities, and in which a A value defined by the following formula (1) is 6.0 to 18.0, and a steel microstructure in which at least 500 particles/mm2 of sulfide of less than 1 μm in equivalent circle diameter and at least 2000 particles/mm2 of sulfide of 1 μm to 5 μm in equivalent circle diameter are distributed.

1 . A free-cutting steel comprising:
 a chemical composition that contains, in mass %,   C: less than 0.09%,   Mn: 0.50% to 1.50%,   S: 0.250% to 0.600%,   O: more than 0.0100% and 0.0500% or less, and   Cr: 0.50% to 1.50%,   with a balance consisting of Fe and inevitable impurities, and in which a A value defined by the following formula (1) is 6.0 to 18.0,
   A value=2([Mn]+2[Cr])/[S]  (1)
 
   where [Mn], [Cr], and [S] respectively denote contents in mass % of elements Mn, Cr, and S; and   a steel microstructure in which at least 500 particles/mm 2  of sulfide of less than 1 μm in equivalent circle diameter and at least 2000 particles/mm 2  of sulfide of 1 μm to 5 μm in equivalent circle diameter are distributed.   
     
     
         2 . The free-cutting steel according to  claim 1 , wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of
 Si: 0.50% or less,   P: 0.10% or less,   Al: 0.010% or less, and   N: 0.0150% or less.   
     
     
         3 . The free-cutting steel according to  claim 1 , wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of
 Ca: 0.0010% or less,   Se: 0.30% or less,   Te: 0.15% or less,   Bi: 0.20% or less,   Sn: 0.020% or less,   Sb: 0.025% or less,   B: 0.010% or less,   Cu: 0.50% or less,   Ni: 0.50% or less,   Ti: 0.100% or less,   V: 0.20% or less,   Zr: 0.050% or less, and   Mg: 0.0050% or less.   
     
     
         4 . A method of producing a free-cutting steel, the method comprising:
 rolling a rectangular cast steel at a heating temperature of 1120° C. or more and an area reduction rate of 60% or more to obtain a billet, the rectangular cast steel having a chemical composition that contains, in mass %,   C: less than 0.09%,   Mn: 0.50% to 1.50%,   S: 0.250% to 0.600%,   O: more than 0.010% and 0.050% or less, and   Cr: 0.50% to 1.50%   with a balance consisting of Fe and inevitable impurities, and in which a A value defined by the following formula (1) is 6.0 to 18.0,
   A value=2([Mn]+2[Cr])/[S]  (1)
 
   where [Mn], [Cr], and [S] respectively denote contents in mass % of elements Mn, Cr, and S, and a side length of a cross section of the rectangular cast steel perpendicular to a longitudinal direction being 250 mm or more; and   hot working the billet at a heating temperature of 1050° C. or more and an area reduction rate of 75% or more.   
     
     
         5 . The method of producing a free-cutting steel according to  claim 4 , wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of
 Si: 0.50% or less,   P: 0.10% or less,   Al: 0.010% or less, and   N: 0.0150% or less.   
     
     
         6 . The method of producing a free-cutting steel according to  claim 4 , wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of
 Ca: 0.0010% or less,   Se: 0.30% or less,   Te: 0.15% or less,   Bi: 0.20% or less,   Sn: 0.020% or less,   Sb: 0.025% or less,   B: 0.010% or less,   Cu: 0.50% or less,   Ni: 0.50% or less,   Ti: 0.100% or less,   V: 0.20% or less,   Zr: 0.050% or less, and   Mg: 0.0050% or less.   
     
     
         7 . The free-cutting steel according to  claim 2 , wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of
 Ca: 0.0010% or less,   Se: 0.30% or less,   Te: 0.15% or less,   Bi: 0.20% or less,   Sn: 0.020% or less,   Sb: 0.025% or less,   B: 0.010% or less,   Cu: 0.50% or less,   Ni: 0.50% or less,   Ti: 0.100% or less,   V: 0.20% or less,   Zr: 0.050% or less, and   Mg: 0.0050% or less.   
     
     
         8 . The method of producing a free-cutting steel according to  claim 5 , wherein the chemical composition further contains, in mass %, one or more selected from the group consisting of
 Ca: 0.0010% or less,   Se: 0.30% or less,   Te: 0.15% or less,   Bi: 0.20% or less,   Sn: 0.020% or less,   Sb: 0.025% or less,   B: 0.010% or less,   Cu: 0.50% or less,   Ni: 0.50% or less,   Ti: 0.100% or less,   V: 0.20% or less,   Zr: 0.050% or less, and   Mg: 0.0050% or less.